Create constants for cli argument names

[jstuczyn]

While this is already applicable to current develop branch, it is mostly concerning #config_files branch. If new arguments were to be added, it's relatively easy to accidentally make a typo in one but not the other.
So say the following:

        .arg(
            Arg::with_name("host")
                .long("host")
                .help("The custom host on which the mixnode will be running")
                .takes_value(true)
                .required(true),
        )

...

    if let Some(host) = matches.value_of("host") {
        config = config.with_listening_host(host);
    }

should be change to something like

       const HOST_ARGUMENT_NAME: &'static str: "host";

        ...

 
        .arg(
            Arg::with_name("host")
                .long(HOST_ARGUMENT_NAME)
                .help("The custom host on which the mixnode will be running")
                .takes_value(true)
                .required(true),
        )

...

    if let Some(host) = matches.value_of(HOST_ARGUMENT_NAME) {
        config = config.with_listening_host(host);
    }

[tommyv1987]

Closing issue, archiving for future planning session - the issues / tasks have moved to a new project board, so we can evaluate shortly.